An embodiment of such a measuring arrangement is described for example in German laid-open application (DE-OS) No. 31 51 798. Therein, the body which is displaceable relative to the measuring arrangement is formed by a scale carrier, the surface of which carries a multiplicity of scale division markings wherein, in each position of the scale carrier, at least one of the markings is passed over by a scanning spot generated by the scanning means. A reference line which is predetermined by the measuring arrangement itself serves as the reference marking element. Such an arrangement is used in any situation which involves determining and measuring off with a high degree of accuracy the relative position of two bodies which can be linearly displaced relative to each other or which can be rotated about an axis relative to each other. Examples in that respect are the tool slides of a machine tool which must be positioned with a high degree of accuracy relative to a workpiece which is clamped in the machine, or the telescope of a levelling instrument which is moved relative to the support stand into different azimuth angle positions, the angular spacings of which have to be ascertained with the maximum possible degree of accuracy.
In all those situations, one of the two bodies, for example the machine frame structure or the support stand, carries the scale carrier and the other of the two bodies, for example the tool carriage or the rotary base of the telescope, carries the scanning device, which is operable to ascertain the number of scale carrier markings which are passed over upon linear displacement or upon rotary movement or to measure the spacings between said markings, and which on the other hand, when a new position is reached, is operable to ascertain the spacing of at least one of the markings from the reference line. The latter aspect serves in particular to achieve the maximum level of resolution capability without at the same time having to use a very high level of density of markings on the scale carrier. If in addition, in respect of all mutually adjacent markings, measurements are made in respect of the spacings of those respective markings from the reference line, in a suitable position, it is possible therefrom to detect with a very high degree of accuracy the spacings of the markings from each other so that in that way the entire scale carrier can subsequently be calibrated with a very high degree of accuracy without having to set particular requirements in regard to the degree of accuracy with which the markings have been applied thereto.
In accordance with German laid-open application (DE-OS) No. 31 51 798, the scanning device includes a light source as a transmitter and a differential photodiode as a receiver, the light source and the photodiode being arranged in mutually opposite relationship on the two sides of the scale carrier so that the scale carrier and the markings thereon are scanned by a transillumination process. It is likewise possible however for the transmitter and the receiver to be disposed on the same side of the scale carrier; in that case, the markings must differ from the areas of the scale carrier around the markings, not in regard to transparency but in regard to the reflection factor.
At any event, in accordance with DE-OS No. 31 51 798, the entire scanning device including the transmitter and the receiver is displaced with an oscillatory reciprocating movement which is independent of a relative movement as between the two bodies, in order to be able to carry out the above-mentioned interpolation and calibration measuring operations. By means of a small auxiliary scale carrier which also moves with the scanning device, and a fixed auxiliary scanning device which scans the auxiliary scale carrier, the movement of the scanning device is monitored and measured in such a way that the spacings of scanned markings on the actual scale carrier from a reference line predetermined by the measuring arrangement, for example from the key or crucial measuring line of the auxiliary scanning device, can be ascertained with a high degree of accuracy.
A further possible use in respect of the measuring arrangement set forth in the opening part of this specification is represented for example by adjusting devices in which a body has to be moved into a precisely defined position, the attainment of which is detected for example by virtue of the fact that the spacing between the marking element on the body and a reference marking element becomes equal to zero. Such an adjusting operation occurs for example in the photolithography process in semiconductor manufacture, wherein a photomask has to be put into a precisely defined position relative to a silicon wafer. In that case, the attainment of the required position is detected by virtue of suitable relative displacement as between the silicon wafer and the photomask causing two adjusting crosses to be brought into alignment with each other, one adjustment cross being disposed on the silicon wafer and the other on the photomask, and one of the adjusting crosses serving as the marking element and the respective other cross serving as the reference marking element. Another option is that a line which is to be found in any case on the body to be displaced, such as for example an edge of the body, serves as a marking element and the body is moved into a position in which the spacing between that line or the edge of the body and a reference marking element disappears, at least when viewed in a projection or viewing direction, that is to say the above-mentioned spacing becomes zero, when the body has reached the position in which it is to be put by way of the adjusting operation. In principle, in order to automate such an adjusting operation, it is also possible to use a suitably modified measuring apparatus of the kind described in DE-OS No. 31 51 798, with a mechanically oscillating scanning device.
However a disadvantage of that known arrangement is that, even with an extremely miniaturized construction in regard to the scanning device, a certain minimum mass has to be mechanically reciprocated, that is to say, it has to be periodically accelerated and decelerated. That results in a maximum scanning frequency which is inadequate at least when a machine carriage or slide or a body to be adjusted into a given position is to be moved at high speed into a precisely defined position or for example a target tracking telescope is to be turned with a rapidly moving target.
In the face thereof, it is an objective of the present invention to provide a measuring arrangement for determining the distance between a displaceable marking element and a reference marking element, which makes it possible to achieve a substantially higher scanning and measuring frequency.